Born at the Royal Palace of Madrid on 20 December 1851, she was the eldest surviving daughter of Queen Isabella II and King Francisco de Asís.

In the tumultuous age of Carlist uprisings and sporadic civil war, Isabel was immediately recognized as the heir presumptive to her mother's throne and as such was made Princess of Asturias.

Historians and biographers attribute Infanta Isabel's paternity to José Ruiz de Arana y Saavedra (1826–1891), a young Spanish aristocratic and military officer.

Ruiz de Arana was known to the queen from palace's inner circles; his father, the Count of Sevilla La Nueva, was usher to ambassadors.

It was with some reluctance that King Francisco de Asís recognized Isabel as his daughter as he would do subsequently with all the children Queen Isabella II bore during their troubled marriage.

[3] On 2 February 1852 Isabella II was making a traditional visit to the Royal Basilica of Our Lady of Atocha, introducing her daughter to the public, when she was stabbed by a mad priest.

Queen Isabella II was preoccupied with her turbulent reign and her private life alternated between periods of great affection towards her children and the distant approach to childhood that was the custom of the time.

Isabel's governess, teacher, and friend was Scottish-born Fanny Calderón de la Barca, well-known author of Life in Mexico.

With only a brother of delicate health ahead of her in the line of succession to the throne, there was great interest in arranging an early marriage for Infanta Isabella that would provide descendants.

For political reasons, Isabella II had to recognize the unification of Italy under the Savoy crown, and in order to compensate her cousins from the Bourbon dynasty of the Kingdom of the Two Sicilies, who were upset at this recognition, the ultra-conservative party at the Spanish court, headed by King Francisco de Asís, convinced the queen to arrange the marriage of their eldest daughter with one of the half-siblings of the recently deposed King Francis II of the Two Sicilies, Prince Gaetan, Count of Girgenti (1846–1871), son of King Ferdinand II of the Two Sicilies and Archduchess Maria Theresa of Austria.

[9] On their way back to Spain, while visiting Emperor Napoleon III and Empress Eugénie at Fontainebleau, they received the news of the Glorious Revolution that cost Isabella II her throne.

[10] Gaetan rushed to enter Spain and fought in defense of the monarchy in the Battle of Alcolea, a defeat that marked the end of the reign of Isabella II, who crossed the border into France with the royal family.

For two years, the couple embarked on a series of trips through Europe, visiting Austria, Germany and England, searching in vain for a solution to Gaetan's health.

In the early summer of 1871, Isabel and her husband stayed in Geneva to join the rest of the Spanish royal family, which had escaped disturbances in Paris.

After their mother Queen Isabella II returned to live permanently in France, Isabel's three youngest sisters were placed under her care, and she provided a good education for them.

Isabel also served as a guide to her young cousin Princess Mercedes of Orléans, who married her brother in 1878 and replaced her as the first lady of the kingdom as the new queen.

Following Queen Mercedes' early death in the same year as her marriage, Isabel chose Archduchess Maria Christina of Austria as her new sister-in-law and promoted her as a wife to her brother.

She was an influential figure throughout the regency of Queen Maria Cristina and gave her widowed sister-in-law support; she became a second mother to the children of her late brother.

One of her most significant public activities was her 1910 trip to Buenos Aires, Argentina, as a representative of the Spanish Crown on the occasion of the centennial celebration of the May Revolution that was considered the starting point of the Argentine War of Independence.

She left most of her jewels to her nephew, and her famous Mellerio Shell Tiara subsequently descended to the current Spanish royal family and is frequently worn by Queen Sofía.