The 127 (Waltham Cross-Victoria) came into being on 26th April 1961 as a direct replacement for the 627 trolleybus, although unlike the 627 it continued beyond Tottenham Court Road to Victoria. Like many lengthy London routes it was operated in overlapping sections: Waltham Cross to Tottenham Court Road and Lower Edmonton to Victoria. The Victoria section came off in September 1968 and the route was withdrawn completely after 23rd January 1970.
The current 127 was introduced between Tooting Broadway and Wallington on 25th April 1981, and was basically a renumbered driver-only version of the former route 77 journeys between Tooting Broadway and Wallington, that were operated completely separately from the main 77. Extension to Purley came in February 1984. The route was one of the first to be contracted out to a non-London Buses operator, passing to London & Country in 22nd March 1986. Since then it has changed hands a number of times, including a disastrous spell with Mitcham Belle where reliability and operating standards soon plummeted to an all-time low - so bad was the situation, that the contract was terminated early and passed in December 2005 to Metrobus, who within a few days were receiving letters from members of the public, thanking them for taking it over.
There was an even earlier 127, which commenced in January 1941 between Morden and South Wimbledon (but the long way round, via North Cheam, Worcester Park, Malden and Raynes Park). This was operated by low-height double-deckers and was not actually a new route, but a renumbering of a previously single-deck one (until 1942 London Transport single-deck routes were numbered in the 2xx series, a policy introduced in 1934 for administrative convenience rather than any discernible benefit to the travelling public). It was withdrawn on 20th August 1958, a victim of the swingeing cuts that followed the disastrous seven-week strike by bus crews, which caused passenger numbers to fall by 20%.