Mackinlay, 52, was charged in 2017 of failing to declare more than ?60,000 ($76,000, 66,600 euros) spending on staffing, hotels and adverts during the race in Thanet, southeast England.
Jurors at London's Southwark Crown Court took more than 50 hours to decide the lawmaker was not guilty.
He reacted by tweeting: "#Notguilty".
Party activist Marion Little, 63, was found guilty of two charges and given a suspended sentence. A guilty verdict would likely have triggered a by-election at a particularly sensitive time for Prime Minister Theresa May, who needs as many Conservative MPs as possible to have a hope of passing her Brexit deal in parliament.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)