Hartline had a banner season in 2012 with the Dolphins, his fourth in the NFL, with 74 receptions for 1,083 yards and seemed headed for free agency before the deal was done.
"It feels great," Hartline told NFL.com.
"It feels great to know hard work and perseverance is worth something. But now, it's about winning games. I think we're on the right path. I think we're close."
The Dolphins are also rumoured to be planning an assault on former Pittsburgh Steelers wide receiver Mike Wallace to give young quarterback Ryan Tannehill some fearsome targets.
"I'm excited to see what happens in free agency," Hartline added.
"We want to win. We're set up so we have cap room to make some moves, and that's one way we can win football games. It's important. And the draft will be important, too. It's exciting."
The Dolphins missed the playoffs last season, finishing second in the AFC East with a 7-9 record. (Reportnig by Ben Everill. Editing by Patrick Johnston)