From 2004 to 2006 Merrell, nicknamed "Mateo," made his way through the motorsports ranks, collecting trophies and accolades along the way. By age 19, he was a South Zone champion and the overall Texas State champion. He was also crowned the Rookie of the Year for the Texas Dwarf Car Association to complete a memorable freshman racing season. He then went into the USRA Modifieds in 2005, and chalked up three more wins and produced seven top-five and 10 top-10 finishes. He also earned Rookie of the Year honors when he finished fourth in the points that season for the Texas Asphalt Modified Series.

At 20, Merrell started his USRA Super Late Model Series career and he continued to be successful in yet another series. He ran the full 2006 season and finished second in the championship standings as he racked up two wins, seven top-five and eight top-10 finishes as well as the Rookie of the Year honors for the series.

The 2007 season marked the beginning of Merrell's relationship with Win-Tron Racing. He was selected to drive the No. 32 Dodge in USAR Pro Cup series and ARCA Racing Series, marking him the primary driver for the entry in both series. The 2007 season also marked Merrell's entry into ARCA Racing Series competition, debuting at Toledo Speedway in May. In his lone ARCA Racing Series start of the year, he qualified 13th and finished 16th.

ARCA Racing competition became a full-time pursuit for Merrell in 2009 as he raced in 10 of the 21 events that season, and it was during that season at the age of 24, Merrell experienced his true breakthrough in racing. In 10 ARCA Racing Series events the rookie driver scored five top-five and six top-10 finishes. Merrell and team showed their promise throughout the season as they captured a pole at Toledo Speedway in May and two runner-up finishes at Berlin Raceway and Pocono Raceway.

Merrell returns to the driver's seat of the No. 24 Orion Motorsports' Chevrolet Impala for his first season in the Southeast Limited Late Model touring series in 2010.