Roberts was born free in Norfolk, Virginia, the second oldest of seven children. His father was said to be a planter of Welsh origin. Joseph's mother Amelia, described as a mulatto who was quite fair, was the planter's slave mistress or concubine. He freed Amelia and her children when she was still young. Amelia gave all of her children but one the middle name of Jenkins, which suggests that may have been the name of their biological father.

Once free, Amelia married James Roberts, a free black. Roberts gave his children his surname and raised them as his own. Roberts owned a boating business on the James River. By the time of his death, he had acquired substantial wealth for an African American of those times.

Joseph Roberts and his siblings by the planter were estimated to be seven - eighths European in ancestry. As the Liberian historian Abayomi Karnga noted in 1926, "He was not really black; he was an octoroon and could have easily passed for a white man." As a boy, Joseph began to work in his stepfather's business, handling goods on a flatboat that transported materials from Petersburg to Norfolk on the James River. The family moved to Petersburg, which was an industrial city. Shortly after the move, the stepfather James Roberts died. Joseph continued to work in his family's business, but also served as an apprentice in a barber shop. The owner of the barber shop, William Colson, was also a minister and one of Virginia's best educated black residents. He gave Roberts access to his private library, which provided much of the youth's early education.

In 1828 Roberts married an 18 year old woman named Sarah. They had an infant child whom they took with them when they emigrated to Liberia, under the auspices of the American Colonization Society. Both Sarah and the child died in the first year in the colony. Some time after his wife's death, Roberts married again, to Jane Rose Waring. She was a daughter of Colston Waring and Harriet Graves, Virginians who had also migrated to the colony.

After hearing of the plans of the American Colonization Society to colonize the African coast at Cape Mesurado near modern day Monrovia, Roberts decided to join an expedition. The restrictions in Virginia on free Negroes played an important part in his decision. The Roberts family was strongly religious and they felt called to evangelize the indigenous people of Africa. On February 9, 1829 they sailed for Africa on the Harriet along with his mother and five of his six siblings. Travelling the same ship was James Spriggs Payne, who would later become Liberia's fourth president.

Once in Monrovia, Roberts and two of his brothers established a business with the help of their friend William Colson of Petersburg. The company exported palm products, camwood, and ivory to the United States, and traded imported American goods at the company store in Monrovia. In 1835, Colson emigrated to Liberia, but died shortly after his arrival. Expanding into coastal trade, the Roberts family became successful members of the local establishment. During this time, Joseph's brother John Wright Roberts entered the ministry of the Liberia Methodist Church, founded by Americans. Later he became a bishop. After starting as a trader, the youngest son, Henry Roberts, studied medicine at the Berkshire Medical School (now part of the University of Massachusetts Medical School) in Massachusetts. Joseph Roberts was successful enough to pay for his brother's studies. Henry returned to Liberia to work as a physician.

In 1833, Joseph Roberts became high sheriff of the colony. One of his responsibilities was to organize militias to travel to the interior to collect taxes from the indigenous peoples and put down their rebellions against colonial rule. In 1839, the American Colonization Society appointed Roberts vice governor. Two years later, after the death of governor Thomas Buchanan, Roberts was appointed as the first mixed race (considered black) governor of Liberia. In 1846, Roberts asked the legislature to declare the independence of Liberia, but also to maintain cooperation with the American Colonization Society. The legislature called for a referendum, in which voters chose independence. On July 26, 1847, Roberts declared Liberia independent. He won the first presidential election on October 5, 1847, and was sworn into office on January 3, 1848, with Stephen Allen Benson as vice president.

Roberts was re-elected three more times to serve a total of eight years, until he lost the election in 1855 to the vice president Benson.

Attempts to found a state based upon some 3000 settlers proved difficult. Some coastal tribes converted to Christianity and learned English, but most of the indigenous Africans retained their traditional religion and language. The slave trade continued illicitly from Liberian ports, but the British Navy finally closed it down in the 1850s.

Roberts spent the first year of his presidency attempting to attain recognition from European countries and the United States. In 1848 he traveled to Europe to meet Queen Victoria and other heads of state. Great Britain was the first country to recognize Liberia, followed by France in 1848 or 1852 (accounts differ). In 1849, the German cities of Hamburg, Bremen and Lübeck recognized the new nation, as did Portugal, Brazil the Kingdom of Sardinia and the Austrian Empire. Norway and Sweden did so in either 1849 or 1863, Haiti in either 1849 or 1864, Denmark in either 1849 or 1869 (accounts differ). The United States withheld recognition until 1862, during the presidency of Abraham Lincoln. Reportedly, the government feared that the southern congressmen would not accept a black ambassador in Washington, D.C.

Maryland Colony declared in 1854 its independence from the Maryland State Colonization Society but did not become part of the Republic of Liberia. It held the land along the coast between the Grand Cess and San Pedro Rivers. In 1856, the independent state of Maryland (Africa) requested military aid from Liberia in a war with the Grebo and Kru peoples. They were resisting the Maryland colonists' efforts to control their trade. Roberts assisted the Marylanders, and a joint military campaign by the two groups of African - American colonists resulted in victory. In 1857, the Republic of Maryland joined Liberia as Maryland County.

During his presidency, Roberts expanded the borders of Liberia along the coast and made attempts to integrate the indigenous people of Monrovia into the Republic. By 1860, through treaties and purchases with local African leaders, Liberia extended its boundaries to include a 600 mile (1000 km) coastline. The settlers built schools and Liberia College (which later became the University of Liberia). During these early years, agriculture, shipbuilding, and trade flourished.

Roberts has been described as a talented leader with diplomatic skills. His leadership was instrumental in giving Liberia independence and sovereignty. Later in his career, his diplomatic skills helped him to deal effectively with the indigenous people and to maneuver in the complex field of international law and relations.

After his first presidency, Roberts served for fifteen years as a major general in the Liberian army, as well as a diplomatic representative in France and Great Britain. In 1862, he helped to found and became the first president of Liberia College in Monrovia, where he served as president until 1876. Roberts frequently traveled to the United States to raise funds for the college. Until his death he held a professorship in jurisprudence and international law.

In 1871, president Edward James Roye was deposed by elements loyal to the Republican Party on the grounds that he was planning to cancel the upcoming elections. Roberts, one of the Republican Party's leaders, won the ensuing presidential election and thus returned to office in 1872. He served for two terms until 1876. While he was incapacitated by illness from 1875 until early 1876, Vice - President Anthony W. Gardiner was acting president.

In the decades after 1868, escalating economic difficulties weakened the state's dominance over the coastal indigenous population. Conditions worsened — the cost of imports was far greater than the income generated by exports of coffee, rice, palm oil, sugarcane, and timber. Liberia tried desperately to modernize its largely agricultural economy.

Roberts died on February 24, 1876, less than two months after his second term had ended. In his testament, he left $ 10,000 and his estate to the educational system of Liberia.

• Liberia's main airport, Roberts International Airport, the town of Robertsport and Roberts Street in Monrovia are named in honor of Roberts.
• His face is depicted on the Liberian ten dollar bill introduced in 1997, and the old five dollar bill in circulation between 1989 and 1991.
• His birthday, March 15, is a national holiday in Liberia.