Gallagher began her singing career in Britain with the National Youth Jazz Orchestra and went on to become a professional actress performing leading roles in plays and musicals across the country.
During her time of acting, Gallagher is known for her role as Ellen Smith in The Bill, for her appearances on radio in No Commitments, Salem's Lot, and Memorials to the Missing. She also voiced Karla the zebra in both the UK and US versions of Mama Mirabelle's Home Movies. Further appearances include the voices of Little Miss Chatterbox, Little Miss Daredevil, Little Miss Naughty (Season 2), and Little Miss Whoops in the UK version of The Mr. Men Show, Sarah in Footballers' Wives, and also Alison Canning in Casualty. Additionally, Gallagher was known for her work presenting the BBC show Playdays.
Gallagher has also provided the voices of some of the letters in CBeebies' Alphablocks, and in the popular Cbeebies animated series The Octonauts as Dashi Dog the photographer, and the voices of Nicole Watterson, Penny Fitzgerald, Teri, and Margaret Robinson in the Cartoon Network show The Amazing World of Gumball, in which she does an American accent. Gallagher does the voice of Mater's Computer in Cars 2. She also voiced KT in The Pinky and Perky Show, Egbert the Badger and Gilda the Pigeon in Poppy Cat, Lapitch in the English version of the 2000 television series of Lapitch the Little Shoemaker, Noddy in the 2009 version of Noddy in Toyland, Dennis's mother and several characters in the 2009 version of Dennis the Menace, Pip in Pip Ahoy!, Lottie's mother and several characters in Frankenstein's Cat, Foofa in the UK dubbed version of Yo Gabba Gabba!, Gerald and several characters in Fleabag Monkeyface, Mrs. Piecrust in Mike the Knight, Hugo in What's the Big Idea (CBeebies), both Gaspard and Lisa's mothers, Lisa's little sister Lila and Gaspard's grandmother Mathilde in Gaspard and Lisa, Lucy Selby in Postman Pat: The Movie, Driver Dottie in Engie Benjy, Miss Ladybird, Bobee, and Millice Ant in The Hive, Darzi in The Jungle Book, Hermoine in Sherlock Yack, Jessica in Dude, That's My Ghost!, Zak and Zou's mother in Zou, Adoraball, Pam, Manageball and several characters in The Likeaballs and various characters such as Emily (UK), Rosie (UK), Belle (UK/US), Daisy (UK/US; Sodor's Legend of the Lost Treasure and The Great Race only), Mavis (UK/US), Annie & Clarabel (UK/US), Judy (UK/US), Lady Hatt (UK/US), and other characters alike on Thomas & Friends.
She has also done redubs for several anime films such as Laughing Target, X, Bounty Dog, Demon City Shinjuku, and Cyber City Oedo 808 and later returned to do voice acting for anime with the English dubbed version of the TV series Ronja, the Robber's Daughter.
In 2010, she voiced Queen Titania, Mrs. Walker, and Fern in the UK version of the movie Rainbow Magic: Return to Rainspell Island. She does the voice of Matilda, Young Stella, and Zara the Cruzeao in Angry Birds Space.
In 2015, she provided voices for several minor characters in Thunderbirds Are Go, and now as the role of the titular character in the UK Version of Henry Hugglemonster and the Voice Trumpets and the Tiddlytubbies in the 2015 reboot of Teletubbies.
She has also done various voice-over work for video games, commercials, radio plays, BBC radio dramas, and audiobooks such as several new Meg Cabot books and the 2007 audiobook adaptation of Charles Dickens's famous novel Bleak House which became The Times audio book of the year. Gallagher has also been a member of the BBC Radio Drama Company and can be well known as a recognizable voice for listeners of Radio 4's Book of the Week and Book at Bedtime.
Gallagher is also the lead singer of a musical band called Arcelia which has been performed in various London pubs, as well as supporting several other UK-based bands.
|2000||Lapitch the Little Shoemaker||Lapitch, Perwinkle Percival III||Sean Barrett (English voice)/George Roubicek (English voice)||English version|
|2007-2017||Mama Mirabelle's Home Movies||Karla the Zebra||Unknown||Actor now Kuchi Braaso.|
|2008–2009||The Pinky and Perky Show||KT||Cyril Adam|
|2009–present||Thomas & Friends||Emily (UK), Rosie (UK; series 13-16), Belle (UK/US), Gina (UK/US), Frieda (UK/US), Marion (UK/US; singing voice), Daisy (UK/US; Sodor's Legend of the Lost Treasure and The Great Race only), Mavis (UK/US), Judy (UK/US), Annie & Clarabel (UK/US), Lady Hatt (UK/US), Bridget Hatt (UK/US), Stephen Hatt (UK/US), The Duchess of Boxford (UK), and a few others||Greg Tiernan/Rob Silvestri/David Baas/David Stoten/Don Spencer/Dianna Basso|
|2011–present||The Amazing World of Gumball||Penny Fitzgerald, Nicole Watterson, Teri, Jackie Wilson, Margaret Robinson and other various female characters|
|2011–present||The Extraordinary Adventures of Poppy Cat||Egbert and Gilda|
|2015–present||Thunderbirds Are Go||EOS and various other female characters||David Scott, Theo Baynton|
|2015–present||Teletubbies||Voice Trumpets, Tiddlytubbies|
|2015–present||Miffy's Adventures Big and Small|
|2006||Rule of Rose||Wendy, Joshua|
|2007||The Witcher||Abigail, Alvin|
- Teresa voiced all female characters for Thomas & Friends for the first few years.