Originally from Exmouth, Devon, Alistair is the youngest of seven children. In 2005, he was one of 35 students from around England chosen to participate in the 20th anniversary celebration of Les Misérables at Queen's Theatre in the West End. In 2008, while a student at Exeter College, he competed in Any Dream Will Do, and made it to the final 20 contestants, although he was not selected to be on the show. However, Any Dream Will Do judge Bill Kenwright asked him to appear in the 2008-2009 UK tour of Joseph and the Amazing Technicolor Dreamcoat, where he played Zebulun and understudied both Benjamin and Joseph.
In 2009, he made his West End debut as Marius in Les Miserables, where he spent 18 months in the role. He said of Les Mis, "I should be sick to death of the show, but I can't get over it. It's my favorite."He also played the role of Jean Prouvaire in the Les Misérables 25th Anniversary Concert at the O2 on October 3, 2010.
He also appeared as Walter and understudied Claude in a European tour of Hair, before he was tapped to play Jean Prouvaire in the 2012 film Les Misérables. Since that time, he also appeared as Billy in the 2012 revival of Taboo at the Brixton Clubhouse, and is currently playing Billy and understudying Albert in the West End production of War Horse. In Novemeber it was announced that Alistair will be playing the role of Chris in the 2014 West End revival of Miss Saigon.