Educated at William Ellis School, Highgate, London, Titmus was in his school's first XI by the age of thirteen, and when sixteen he wrote to Lord's, the ground being close to his home, to ask for a trial. He was accepted on to the groundstaff after bowling only a few balls, and in June 1949 he made his first-class cricket debut for Middlesex against Somerset at Bath, at the age of 16 years and 213 days, Middlesex's youngest cricketer ever at that point.

1950 was Titmus's first full season of county cricket, and he performed reasonably well, taking 55 wickets including 7–34 against Minor Counties in July. His appearances in 1951 and 1952 were restricted because of his National Service obligations, although he played for the Combined Services. In 1953 he returned to play for Middlesex full-time, and took 105 wickets, the first of 16 years in which he would reach three figures.

1955 was a good year for Titmus, as he did the double for the first time: he took what would remain his best season's haul of 191 wickets at just 16.31, taking five or more wickets in an innings on no less than 18 occasions. 158 of these wickets were for his county, beating by four the record set up in 1900 by Albert Trott. He also passed a thousand runs for the first time, scoring 1,235 including the first of his six centuries, making 104 against Hampshire albeit in a losing cause as Middlesex lost by an innings.

A fine display for MCC against the South Africans, where he took 8–43 in the second innings, brought Titmus his England debut for the second Test at Lord's, but he took only one wicket (that of Hugh Tayfield) and failed twice with the bat; and had a worse time in the third Test at Old Trafford, making 0 and 19 and taking 0–51. He was dropped,^[2] but he went on a non-Test tour to Pakistan with MCC that winter.

From 1956 to 1962 inclusive, Titmus achieved the double in every year except 1958, but a place in the Test team still eluded him. 1961 was his best year with the bat, as he scored 1,703 runs at a fine average of 37.02, including 14 half-centuries; he passed 50 more than a hundred times in the course of his first-class career.

His form in 1962, 136 wickets and 1,238 runs, led to Titmus being recalled to Test cricket, and he played in the third and fourth Tests against Pakistan. For his performances that year (including a career-best nine for 52 against Cambridge University) he was made one of the Wisden Cricketers of the Year in the 1963 edition of the Almanack. Titmus went to Australia for the 1962-63 Ashes series and made his highest first-class century of 137 not out vs South Australia. He played in all five Tests, and took more wickets than any other English bowler; 21 at 29.33, including a Test career best 7 for 79 in the Third Test^[2] and 5 for 103 in the Fifth, both at Sydney, and making 59 not out in the Fourth Test at Adelaide.

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