The New Avengers was developed by original Avengers producers Albert Fennell and Brian Clemens as a sequel series, produced jointly by the UK, France and Canada and costing £125,000 per episode to make at Pinewood Studios.

Patrick Macnee was 53 when production began, raising doubts about whether he could handle stunt and action work; this partly motivated adding a second, younger partner (Gambit) alongside Purdey, and Macnee lost weight during the show to keep up athletically with his co-stars.

Joanna Lumley's Purdey character drew on her real background as a former Royal Ballet trainee, which she said explained the high-kicking fighting style she used throughout the series.

An attempt to bring back Diana Rigg as Emma Peel for a cameo was unsuccessful, so the producers instead reused old footage of her from two 1960s Avengers episodes for a phone-cameo in 'K Is for Kill Part One', with her voice dubbed by Sue Lloyd.

Ian Hendry, who had played Steed's original 1960s partner David Keel, returned to guest-star in a different role in the episode 'To Catch A Rat'.

The episode 'Obsession' guest-starred Martin Shaw and Lewis Collins, both stars of Brian Clemens and Albert Fennell's other action series The Professionals.

Some New Avengers storylines were recycled from earlier scripts: 'Medium Rare' was based on the Thriller episode 'Murder in Mind', and 'Gnaws' (its title parodying the film Jaws) was based on the Thunderbirds story 'Attack of the Alligators!'

To secure financing for a full 26-episode run, production company Nielsen Ferns joined the project on condition of promoting Canada, so the final four episodes were shot in Toronto (with 'Forward Base' filmed partly at Ward's Island) and were captioned 'The New Avengers in Canada'.

Brian Clemens was heavily committed to The Professionals during production of the Canadian episodes, so control passed to a largely local crew, and the resulting episodes drew heavy criticism from both fans and Clemens himself.

Composer Laurie Johnson, who wrote the original Avengers theme from 1965 onward, returned to create an updated theme for The New Avengers that still opens with the same fanfare as the original.

Financial problems ended plans for a third series, and two later attempts in 1979 and 1980 to revive the show failed because co-financing arrangements could not be agreed, despite strong sales including to CBS in the US.

Brian Clemens wrote a pilot called Escapade for Quinn Martin's QM Productions, featuring Granville Van Dusen and Morgan Fairchild as equivalents of Gambit and Purdey; broadcast on CBS in 1978, it was not picked up as a series.

In 1994, strong-than-expected videocassette sales after Joanna Lumley and Gareth Hunt publicised the release led Brian Clemens to consider reuniting them for a spin-off series; a script was written but plans stalled for undisclosed reasons.

When the BBC reran the series in 1995/96, prints for several early episodes supplied by Canal Plus were deemed not broadcast quality, so the final four episodes were actually shown first while better prints of the others were sourced, resulting in notable picture and audio quality variations until a full 4K restoration from the original 35mm negatives was released in 2025.