Short answers, pulled from the story.
George Martin was called the fifth Beatle because of his extensive involvement in every original Beatles album, where he wrote most of their orchestral and string arrangements and played piano or keyboards on numerous tracks. In 2016, Paul McCartney wrote that "if anyone earned the title of the fifth Beatle it was George." Julian Lennon also called Martin "the fifth Beatle, without question."
George Martin produced 30 number-one hit singles in the United Kingdom and 23 number-one hit singles in the United States across his career. He also produced 16 number-one albums in the UK and 19 number-one albums in the US.
George Martin informed EMI in June 1964 that he would not renew his contract, and he left the company in August 1965. He had repeatedly sought a royalty on record sales and been refused; he departed after EMI's managing director inadvertently revealed that EMI had made £2.2 million in net profit from Martin's records in 1964 alone.
George Martin served as arranger throughout most of Sgt. Pepper's Lonely Hearts Club Band, which was released in early June 1967 and cost £25,000 to produce. He arranged Indian and Western classical elements for "Within You Without You", instructed a 45-piece orchestra to play the 24-bar climax in "A Day in the Life", and played piano on "Lovely Rita", harpsichord on "Fixing a Hole", and multiple instruments on "Being for the Benefit of Mr. Kite!"
Before working with the Beatles, Martin was head of EMI's Parlophone label from 1955, where he produced comedy and novelty records with artists including Peter Sellers, Spike Milligan, and Bernard Cribbins. He also produced the Beyond the Fringe cast album in 1961, scored Britain's first skiffle hit when he signed the Vipers Skiffle Group in 1956, and achieved his first British number-one in May 1961 with the Temperance Seven's "You're Driving Me Crazy."
George Martin died on the 8th of March 2016 at his home in Coleshill, Oxfordshire, aged 90. He was knighted in 1996, inducted into the Rock and Roll Hall of Fame in 1999, and won six Grammy Awards across his career, including the 1993 Grammy Award for Best Musical Show Album for the Broadway production of The Who's Tommy.