Mercado was originally drafted by the Alaska Aces as the fifth pick in the 2008 PBA draft. However, he was immediately traded to the Rain or Shine Elasto Painters, together with Eddie Laure for Joe Devance and the 2009 and 2010 second round picks of Alaska.
With all the focus on Gabe Norwood, that year's first pick overall, and not much hype he scored a record 29 points in his rookie debut game, the highest since Asi Taulava scored 32 points for Mobiline in his debut. Since then, he slowly became one of the PBA's elite point guards.
If not for a hamstring injury that sidelined him in a couple of games into their semifinal series against the Barangay Ginebra Kings, Rain or Shine would have gone farther in the Fiesta Conference. A member of the All-Rookie Team, Mercado finished second to teammate Gabe Norwood for Rookie of the Year honors. Mercado emerged as the team's top scorer with 13.5 points per game and his 4.5 assists per night did not only lead the team but was also number 6 overall among locals. The only glitch in his rookie season was his 2.9 turnovers per game which ranked seventh overall and led the rookies in that category.
He has been the team's cornerstone as he always led the Painters to winning games. Against the Coca-Cola Tigers in a knockout game, he scored 28 points to lead them. But unfortunately they would end up short after losing to the Purefoods Tender Juicy Giants.
He averaged 17.7 points in the eliminations then had a 45-point outburst against the B-Meg Derby Ace Llamados. In another knock out game against the Tigers, he delivered crucial plays, despite fouling out, in a come from behind win. Against the Llamados in Game 3, he almost recorded a triple double with 16 points 15 assists 9 rebounds. But, in Game 5 of the series, they lost to the Llamados.
Before the start of the 2011 PBA Commissioner's Cup, he and Jay-R Reyes were among the eight players involved in the three-way trade between Meralco, Rain or Shine and Air21 in what was easily one of the biggest trades that happened in that season. Mercado, along with Paolo Bugia were shipped to the Bolts for Beau Belga. He was still able to perform the same way he did with the Elasto Painters. However, it was not enough for the Bolts to past the eliminations of the latter conference.
Before the start of the 2013 PBA Commissioner's Cup, he along with Kelly Nabong, Jaypee Belencion and Yousif Aljamal were traded to GlobalPort Batang Pier for Rey Guevarra, Vic Manuel, Josh Vanlandingham and 2015 first round pick.
Before the start of the 2014 PBA Commissioner's Cup, Mercado was traded to San Miguel in exchange for Alex Cabagnot. At the start of 2014 PBA Governors' Cup, Mercado changed his jersey number from 0 to 7.
Before the 2014–15 PBA Philippine Cup trade deadline, just before the playoffs, Mercado was traded back to GlobalPort, together with San Miguel's 2018 and 2019 second round picks, in exchange for Cabagnot.
On January 8, 2015 Mercado was traded again by GlobalPort, this time to Barako Bull which gave up Denok Miranda and a 2016 second round pick.
On May 4, 2015 before the start of 2015 PBA Governors' Cup Mercado was traded to Barangay Ginebra in exchange of Joseph Yeo. Mercado promised that he will do anything for Ginebra because he says he is in his best shape of his career. Mercado debuted for Barangay Ginebra on May 8, 2015 against the team that drafted him, the Alaska Aces, but his new team lost.
Mercado is previously engaged to actress Denise Laurel. However, on October 20, 2016, a day after Ginebra won their first title after 8 years, Laurel announced through her Instagram account that they have ended their relationship.
Correct as of October 19, 2016
Mercado is also a member of the Philippine National Basketball team, Smart Gilas. He was personally handpicked by Rajko Toroman alongside Kelly Williams and Asi Taulava. He played for the Philippines in the 2010 Asian Games and the 2013 William Jones Cup.