Millar was signed for £5000 by Rangers on 12 January 1955 from Dunfermline, where he played at half back, and played in that position on a number of occasions in the early years of his spell at Ibrox. He achieved success however, when, after filling in as centre forward in a pre-season tour of America and scoring regularly, he became an integral part - at centre forward - of the Rangers team of the early 1960s, with players such as Bobby Shearer, Eric Caldow, John Greig, Jim Baxter and Alex Scott and as part of a highly successful strike partnership with Ralph Brand. Millar's total of 30 goals in the Scottish Cup is a post-war record for Rangers, equalled only by Derek Johnstone, and on two occasions he scored twice in Scottish Cup finals, in 1960 and 1964, another record equalled only by Johnstone. Millar was the first ever substitute to come on for Rangers, replacing Jim Forrest during a 5-0 victory over Falkirk in a league match at Ibrox in October 1966.
Millar was also known for his scoring record against Rangers' arch-rivals Celtic, with 13 goals in Old Firm games, including two winning goals at Celtic Park in the traditional New Year's Day match - in 1960, and 1964.