Jenny worked at the Watermill Theatre, Newbury, Berkshire in 1982, returning periodically, to assist stage management, drive the van, and lead sing-alongs.

She has received numerous awards and nominations, winning the 1999 Olivier Award for Best Supporting Actress in a Musical for her performance in Mamma Mia!. She had previously won the award seven years earlier for her portrayal of the character Luce in George Abbott's The Boys from Syracuse.

In the 2001 ITV children's TV series Weirdsister College (a sequel to The Worst Witch which follows the adventures of schoolgirl witch Mildred Hubble into further education), Jenny Galloway portrayed the college porter known as the Beetle. The Beetle was strict, widely disliked and a stickler for the rules to the point where she actually seemed to enjoy evicting people to save money!

Film credits include In Transit, About a Boy, Fierce Creatures and the role of the Foreign Secretary in Johnny English. She appeared in Madame de Sade alongside Judi Dench and Deborah Findlay as Charlotte in 2008.

She also reprised her role as Madame Thénardier in the 2010 25th anniversary concert of Les Misérables at the O2 arena, London in October 2010. This concert is now available on DVD and Blu-ray. She was briefly reunited with her former co-star Alun Armstrong at the end of the performance when he appeared alongside the rest of the original cast of the musical for the finale. She and Armstrong had appeared as the Thénardiers in the 10th Anniversary Concert at the Royal Albert Hall. Galloway also played the character in 2006 Broadway Revival of the show.

In 2013 she appeared as Sister Thomas in the Father Brown episode "The Bride of Christ".