Maclin was considered a top National Football League prospect following his second season at Missouri, and roommate Sean Weatherspoon hoped to convince Maclin to return for his junior year instead of leaving Missouri for the 2009 NFL Draft. Weatherspoon's efforts were unsuccessful, and on January 9, 2009, Maclin declared for the 2009 NFL Draft. Draft analyst Mel Kiper Jr. projected Maclin to go to the Oakland Raiders with the tenth pick in the first round of the NFL Draft, but that was after Michael Crabtree, another highly regarded prospect at wide receiver, measured in at 6'1", two inches shorter than his published height. Crabtree also suffered a broken bone (Jones fracture) in his left foot prohibiting him from working out at the NFL Combine. Maclin's personal best in the 40-yard dash was 4.31 seconds, but he wanted to lower that to 4.29 seconds in the NFL Scouting Combine in Indianapolis. He tripped and fell during a passing drill at the NFL Scouting Combine on February 22, 2009, hyperextending his left knee, and wasn't running at full speed, but previously to his fall, unofficially ran the 40-yard dash in 4.34 seconds and 4.4 seconds that morning. He ran the 40-yard dash at the NFL Scouting Combine in 4.45 sec., but the earlier unofficial times conflicted with other reports that said he ran the dash first in 4.34 seconds and then 4.44 seconds. After originally believing he had to get an MRI on his injured left knee, it was reported he would not need one as the knee was healthy by the end of the Combine. He also had a sore right hip because of the fall, but that also proved to be minor. Maclin scored very high for a wide receiver on the Wonderlic Test with a score of 25, compared to the normal score of 18 for wide receivers.
Maclin started in his first NFL game on September 27, 2009, against the Kansas City Chiefs in week 3 in place of the injured Kevin Curtis. He caught four passes for 33 yards in that game.
On October 11, 2009, Maclin scored his first NFL career touchdown on a 51-yard reception from Donovan McNabb as the Philadelphia Eagles beat the Tampa Bay Buccaneers 33-14. Maclin also recorded his second NFL touchdown reception later in the game.
On December 27, 2009, in a home game against the Denver Broncos and with the score tied 27–27, Maclin made a decisive catch on the sideline to extend a crucial late drive for the Eagles. McNabb threw a tight spiral to the sideline and Maclin caught it while toeing the line. Initially the catch was ruled incomplete, but upon further review was ruled a catch and first down. Three plays later, Eagles kicker David Akers kicked the game-winning field goal, sealing victory for Philadelphia by a score of 30–27. Maclin finished with 6 catches for 92 yards. For this he received Rookie of the Week honors.