Born at the now destroyed Ribeira Palace in Lisbon, her full name was Maria Francisca Isabel Josefa Antónia Gertrudes Rita Joana. On the day of her birth, her grandfather, King John V of Portugal, created her the Princess of Beira. One of four children, she was the eldest of 4 daughters. When her father, Joseph I, succeeded to the throne in 1750, Maria was declared his heiress and given the traditional title of Princess of Brazil, though not Duchess of Braganza.

Maria would grow up in a time when her father's government and country was governed completely by the famous Marquis of Pombal. Her father would often retire to the Queluz National Palace which was later given to Maria and her husband. The Marquess took control of the government after the terrible 1755 Lisbon earthquake of November 1, 1755, in which around 100,000 people lost their lives; the palace of her birth was also destroyed in the disaster.

After the earthquake, her father was often uncomfortable at the thought of staying in enclosed spaces and later had claustrophobia. The king later had a palace built in Ajuda, away from the city centre. This palace became known as Real Barraca de Ajuda (Royal Hut of Ajuda) because it was made of wood. The family would spend much time at the large palace and it was the birthplace of Maria's first child. In 1794 the palace burned to the ground and Ajuda National Palace was built in its place.

Infanta Maria married her uncle, Infante Peter of Portugal (July 5, 1717 - May 25, 1786) on June 6, 1760 when the bride was 25 and the groom was 43. Despite the age gap, the couple had a happy marriage. Peter automatically became co-monarch (as Peter III of Portugal) when Maria ascended the throne, as a child had already been born from their marriage. The couple had six children.

In 1777, she became the first undisputed Queen regnant of Portugal, and the Algarves, and the 26th (or 27th according to some historians) Portuguese monarch. Her husband became her co-ruler as Peter III. Despite the couple being co-rulers, the power was always to remain with Maria who was a good ruler prior to her madness.

Her first act as queen was to dismiss the popular prime minister, the Marquis of Pombal, who had broken the power of the reactionary aristocracy via the Tavora affair, partially because of Pombal's Enlightenment, anti - Jesuit policies. Noteworthy events of this period were Portugal's membership of the League of Armed Neutrality (July 1782) and the 1781 cession of Delagoa Bay from Austria to Portugal.

Queen Maria suffered from religious mania and melancholia. This acute mental illness (perhaps due to porphyria, which also may have tainted George III of the United Kingdom) made her incapable of handling state affairs after 1792. Her madness was first officially noticed in 1786 when Maria had to be carried back to her apartments in a state of delirium. The queen's mental state became increasingly worse. The year of 1786 saw her husband lose his life in May. Maria was devastated and forbade any court entertainments and according to a contemporary, the state festivities resembled religious ceremonies. Her state worsened after the death of her eldest son, aged 27, from smallpox, and of her confessor, in 1791. After the end of 1791, her mental state seemed to be turning to even worse. In February 1792, she was deemed as mentally insane and was treated by Francis Willis, the same physician who attended George III of the United Kingdom. Willis wanted to take her to England, but that was refused by the Portuguese court. The young prince John took over the government in her name, even though he only took the title of Prince Regent in 1799. When the Real Barraca de Ajuda burnt down in 1794, the court was forced to move to Queluz where the ill queen would lie in her apartments all day and visitors would complain of terrible screams that would echo throughout the palace.

In 1801 the Spanish dictator Manuel de Godoy invaded Portugal with backing from Napoleon, but was forced to abandon the campaign in the same year. However the Treaty of Badajoz on June 6, 1801 forced Portugal to cede Olivenza and part of Guyana to Spain.

The refusal of the Portuguese government to join the Continental Blockade of Britain culminated in the 1807 Franco - Spanish invasion led by General Junot. The General was appointed governor of Portugal pending Napoleon's decision on its ultimate fate.

At the urging of the British government, on 29 November 1807, the entire Braganza dynasty decided to flee to Brazil to establish a Cortes - in - exile, in the Kingdom of Brazil. Along with the Royal Family, she was transported aboard the nau Príncipe Real; during her move from the Royal palace to the docks she was heard screaming throughout the trip, in the middle of the crow and in the carriage. The Queen's dementia was so great that she feared that she was going to be tortured and/or robbed, during her movement by her loyal servants.

In January 1808, Prince John and his court arrived in Salvador, where he signed a commercial regulation that opened commerce between Brazil and friendly nations, which in this case represented England. This important law broke the colonial pact that, until then, only allowed Brazil to maintain direct commercial relations with Portugal.

On August 1, 1808, the British General Arthur Wellesley (later Duke of Wellington) landed a British army in Lisbon and thus initiated the Peninsular War. Wellesley's initial victory over Junot at Vimeiro (August 21, 1808) was wiped out by his superiors in the Convention of Cintra (August 30, 1808). Nevertheless, Wellesley (now Lord Wellington) returned to Portugal on April 22, 1809 to recommence the campaign. Portuguese forces under British command distinguished themselves in the defence of the lines of Torres Vedras (1809 – 1810) and in the subsequent invasion of Spain and France.

In 1815, the regency government elevated Brazil to the status of a kingdom, and Maria I was proclaimed the Queen of the United Kingdom of Portugal, Brazil and the Algarves. When Napoleon was finally defeated in 1815, Maria and her family remained in Brazil.

Incapacitated, she lived in Brazil nine years, always in a unhappy state; the Queen died at a Carmelite convent in Rio de Janeiro on 1816 at the age of 81 (the Prince Regent succeeded her as King John VI of Portugal and Brazil). Her body was returned to Lisbon, and interned in a mausoleum in the Church of Estrela (Portuguese: Igreja da Estrela), that she had helped found.

Later, a marble statue of the Queen was erected in National Library in Lisbon, by the students of Joaquim Machado de Castro, who directed the project.