The 19-year-old attacker, who recently signed an extended contract with the Blues, will join Ballymena until the end of the season.
Frazer had a similar spell at Warrenpoint Town last season and United boss Glenn Ferguson hopes he can boost the Sky Blues’ squad as the campaign reaches its run-in.
“Jonny can play wide right or up front and he has great pace,” Ferguson told Times Sport.
“I’ve watched Linfield Swifts a few times and we’ve seen him a few times when he’s come on for the first team.
“Our coach Sammy McFadden was his manager at Linfield Swifts last year and he raves about him.
“He can stretch defences and gives us another option,” added Ferguson.
Frazer is likely to make his first appearance in Friday night’s friendly against Derry City at the Showgrounds (7.45pm).
With United having no fixture on Irish Cup weekend, Ferguson agreed to a request from new Derry boss Kenny Shiels for a match, as they continue their preparations for the start of the new Airtricity League season.
Admission prices are £7 for adults and £5 for senior citizens and U-16s.
Primary 7 pupils will be admitted to Friday night’s game for free under an initiative involving local schools.
* United report no bids for transfer-listed striker Darren Henderson and the frontman will now stay at the Showgrounds until the end of the season.