Melvin started 27-of-41 games played at Northern Illinois, totaling 172 tackles (117 solo), six interceptions, 3.5 tackles for loss, one forced fumble and 35 passes defensed. His 35 passes defensed ranked second in school history. He saw action in two games as a walk-on freshman in 2009 and was later awarded a scholarship during the spring of 2010. He saw action in all 14 games (two starts) as a sophomore in 2010, compiling 39 tackles (24 solo), 1.0 tackle for loss, four passes defensed and two interceptions. He started all 13 games as a junior in 2011, recording 78 tackles (54 solo), 1.0 tackle for loss, nine passes defensed and three interceptions. He started all 12 games as a senior, recording 55 tackles (39 solo), 1.5 tackles for loss, 17 passes defensed, one interception and one forced fumble. He led the Mid Atlantic Conference and set the single-season school record with 17 passes defensed.
Melvin was originally signed by the Tampa Bay Buccaneers as an undrafted rookie free agent on May 2, 2013. He was placed on Injured Reserve by the Buccaneers on October 15, 2013. He was released by Tampa Bay on September 15, 2014.
Melvin then signed to the Miami Dolphins practice squad. Melvin was later signed by Baltimore on November 8, 2014 off the Dolphins' practice squad after they suffered several injures at the defensive back position.
Melvin made his debut for the Baltimore Ravens on December 14, 2014. In his first game, he was a pleasant surprise on the Ravens' hobbled secondary, recording 5 tackles and 1 pass deflected. In his first start the next week, Melvin continued his success, recording 8 tackles and a forced fumble. In week 17 against the Cleveland Browns, Melvin had 4 tackles.
However, this success would not carry into the playoffs. On January 10, 2015 against the New England Patriots, Melvin allowed 12 catches on 15 passes thrown his way, giving up 224 yards and two touchdowns, according to Pro Football Focus. After a bad performance in Week 3 of the 2015 NFL Season against the Cincinnati Bengals, he was benched and became inactive for the next two games; the Ravens released Melvin on October 13, 2015.
Melvin was claimed off waivers by the New England Patriots on October 14, 2015. Melvin played in eight games with the Patriots and accumulated 3 tackles and 2 special teams tackles. Melvin was released by the New England Patriots on December 9, 2015, and was re-signed to their practice squad the next day. On January 26, 2016, Melvin signed a futures contract with the New England Patriots.
On July 30, 2016, Melvin was signed by the Dolphins. On September 3, 2016, he was released by the Dolphins as part of final roster cuts.
On September 7, 2016, Melvin was signed by the Indianapolis Colts.