Nomad, hedonist, jack-of-all-trades, and cynic, Shearer was born in Glasgow. He had started (but it is not certain he graduated) several degrees, having studied Chemical Engineering at University of Edinburgh, Philosophy and Law at University of Kent, Education at University of Cambridge and Zoology at University of Leeds. For many years he lived an itinerant lifestyle in countries such as Ethiopia, Kazakhstan, Thailand, Vietnam, Brazil and Nigeria, and is a skilled linguist, speaking Portuguese, French and Spanish. Whilst having spent much time as a teacher, he also worked as a professional psychic, barman, sewer digger, zoo-keeper, dog trainer, RSPB officer, banker and tarot card reader. He has taught rifle shooting to delinquents and carried out refuge work for the Lumholtz's tree kangaroo. He also considered becoming a monk, but admits that "the monks didn't want me." Shearer is currently resident in Beijing where he copy-edits for Xinhua News Agency.
Shearer has been engaged at least five times.
A self-confessed loner, at the time of the program, Jonathan was living on the island of Benbecula in Scotland, cleaning hotel rooms to support his poorly paid ornithology work for the RSPB. He says: "I have a strong dislike for the kind of deeply earnest, green, touchy-feely idiots whom I suspect you are going to populate your island with."
Jonathan made it to the grand final on 27 May and won Castaway 2007.
During the filming of Castaway, Jonathan
- Was booted out of the communal sleeping hut and moved into the potting shed, creating his own little home there.
- Gave away his food rations and warm clothing to those going on the treacherous fishing trips
- Was at the centre of controversy for many episodes after he failed to help Hassan Kobeissi on a joint mission that sparked fury when the other Castaways found out.
- Insulted Wendy Mitchell about her culinary skills, despite her cooking for practically the whole time of their stay on the island.
- Concealed the location of a water well to all but 'Lil Al', after there was a water shortage due to Hassan Kobeissi breaking the water tank.
- Should have been voted out by fellow Castaways in the third week when he and Jason Ross gained the most votes in a vote-out. The production team, unbeknown to the television audience, allowed Lucinda King to change her vote hours after she had decided to vote for Jonathan to leave the island permanently. As a result, Jason had to leave with a 1-vote majority 7-6.