Our family has a wedding in RI June 2018 and I saw there was a marathon in Lubec Maine that I could pair with the trip (I'm going for all 50 states so I need to do stuff like this on vacations). Anyway, we are spending a few days up north then want to stop by Acadia NP before going towards Boston and RI.

My question is, is a day enough to see Acadia? I was thinking Sunday afternoon there and Monday late afternoon or maybe leave Tuesday and head to Boston.

You can definitely make a day work in Acadia, would try for something like this:
Cadillac Mountain sunrise or sunset
Ocean Path (Sand Beach, Thunder Hole, Monument Cove, and Otter Cliff)
Walk some of the Carriage Roads around Jordan Pond + lunch at Jordan Pond
Atlantic Brewing for beers

Would also recommend this hike near Lubec:
Cutler Coast Public Reserved Land http://www.maine.gov/cgi-bin/online/doc/parksearch/details.pl?park_id=44

Good to know thanks.

I assume you've checked this but it's quite a drive from Lubec to Boston. The Maine coast is loooooong (you can go inland for part of it and make it faster but driving along 1 is pretty)

Yeah we have a few stops in mind in between. Going with my parents and my mom loves lighthouses so that's a big plus for her. We also might hop on a train in Brunswick and ride that into Boston to ditch the rental car.