Mendoza's first professional fight was against Harry the Coal-heaver, where they fought 40 rounds and Mendoza's victory won him five guineas. This is also where he received the nickname "The Star of Israel." His quick notoriety attracted the attention of the Prince of Wales and he became the first boxer to have royal patronage.
Despite his burgeoning fighting career, he continued to work ordinary jobs he also continued getting into fights with customers, although he always felt justified in defending himself.
Mendoza stood only 5'7" and weighed only 160 pounds, but went on to win the middleweight championship and also became the lightest heavyweight championship in history.
In 1787 Mendoza defeated Sam Martin ("The Bath Butcher") and cemented his reputation as a first class fighter. He was immediately such a hugely popular that he was "brought back to London by a vast horde of jubilant supporters who carried lighted torches and sang 'See the Conquering Hero Comes' all the way home."
Some of his most famous fights were the rubber match trilogy against his mentor and former trainer (and "most hated rival"), Richard Humphries, where after losing the first fight on January 9, 1788 in just 15 minutes, he went on to "thoroughly dominate" Humphries in the next two fights.
Boxing in the 1700s was a far different affair then it is today. Men fought without gloves and were only given 30 seconds to rest between rounds. A "round" lasted until one of the competitors ended up on one knee or on the ground. Men often died in the ring.
Such a star was Mendoza that he began holding public exhibitions to train London society on how to box and soon began making theater appearances to demonstrate his boxing prowess. In 1789, rather than reporting on the French Revolution the press focused on one of his fights.
Mendoza was considered the father of scientific boxing as he introduced new boxing moves, like using a guard, sidestepping and hitting with a straight left, and he used speed and foot movement and not just brute strength boxing to avoid the damage at the hands of much larger men.
In 1794 Mendoza fought world champion, Bill Warr, at Bexley Common, and became heavyweight champion, despite the size difference. He lost his title in April 15, 1795 to John "Gentleman" Jackson (4 inches taller and 40 lbs heavier) in what was deemed a dirty fight where his opponent "grabbed a handful of Mendoza's long hair, held him, and beat him senseless in the ninth round."
After losing the title he retired and took on a variety of other occupations including becoming the landlord of a pub; giving boxing demonstrations; appearing in the theater in a pantomime, "Robinson Crusoe (or Friday Turned Boxer)"; opening a boxing academy; and working as a recruiting sergeant for the army, as well as a process server.
During this period he also authored two books The Art of Boxing (1789) and The Memoirs of the Life of Daniel Mendoza (1816). Despite all of the revenue he had generated through various occupations and books during this period he was so broke that he ended up in debtor's prison.
His last recorded fight was fought for financial reasons on July 4, 1820, where he was soundly defeated by Tom Owen. He died penniless at the age of 70, leaving his wife Esther in poverty.
Owing to his many "scientific" advancements and his titles, despite his small stature, Mendoza is generally regarded as one of the greatest fighters in boxing history. Historian Pierce Egan said of Mendoza that he was "a complete artist" and "a star of the first brilliancy."