Fast forward two months and the couple's romance seems to be going from strength to strength.
On Saturday, the actress – who separated from Jake Gyllenhaal in December - and her man were seen shopping together in Santa Monica with Reese' two children with ex husband Ryan Phillippe, Ava, ten and six-year-old Deacon.
The low-key outing included a visit to an LA sports store where Jim helped mini athlete Deacon – dressed in blue Panthers strip – chose a new ball and pop-up goals.
With essential soccer equipment picked up, the happy quartet headed out later that same day to join a star-studded gathering at Jennifer Garner's house.
It was the Alias star's 38th birthday, and to mark the occasion Jennifer and her husband Ben Affleck held a special celebration at the home they share with their two children, four-year-old Violet and her little sister, Seraphina, born in January 2009.
Joining Reese, Jim, Ava and Deacon at the affair were Spiderman star Tobey Maguire and his jewellery designer wife Jen Meyer, Hancock actor Jason Bateman and his spouse Amanda, as well as Molly Sims, Jon Favreau, chat show host Jimmy Kimmel and Laurence Fishburne.
Keen to get started on the festivities, Jennifer began her birthday weekend on Friday with a trip to a spa in Pacific Palisades with her elder daughter Violet, where mother and daughter enjoyed a pampering pedicure session together.