In an e-mail, he said he decided to resign as chairman and board member in mid-February but did not explain why he did not quit the board until Monday.
The internal turmoil at the fairgrounds, which draws crowds for year-round equestrian events and 100,000 each spring for its signature horse show, came to light last fall when a board member accused McDevitt of keeping the group in the dark while he worked to develop a high-end hotel and retail center on land he owns beside the fairgrounds.
McDevitt has denied any wrongdoing.
The proposal also called for the horse show to lease several properties to the developer, Urban Outfitters, a company that was founded by Wendy McDevitt's brother-in-law.
She is president of Terrain, an Urban brand that hoped to open a store at the development. McDevitt's site-selection firm was Urban Outfitters' primary real estate broker for the last four years.
There has been no public movement on the proposal - which could transform a dull stretch of Lancaster Avenue and give Devon its first taste of a town center - in more than five months.
During that time, Wade McDevitt was replaced as the show's president at its annual meeting in December and resigned as chairman last month.
In early March, horse show attorney William Lamb said there were still unresolved issues related to the couple's involvement in the development but didn't elaborate, saying he hoped to have an "amicable resolution" soon.
Vince Donohue, another lawyer at Lamb's firm, declined to say Tuesday whether the resignations were that resolution.
"To the extent there was a conflict, it doesn't exist currently," Donohue said. "We have no interest in trying to characterize his decision."