"Watch Me Do" was written by Meghan Trainor along with Jacob Kasher, Gamal Lewis and Ricky Reed, who also produced the song. It is an upbeat, brass-heavy track where Trainor references a number of rap songs, containing "90s hip hop throwback vibes" according to MTV News.
"Watch Me Do" received mostly positive reviews from contemporary music critics. Colin Stutz of Billboard praised the song, noting the presence of "swagger" and "groove", and writing "she opens the track singing over a funky drum beat before going on to gloat". Entertainment Weekly's Dana Rose Falcone gave the song a positive review, describing it as a "female-empowerment" track, adding that she "peppers her new song with some hip-hop and doesn't play the modesty card". Emilee Lindner of Fuse dubbed the song as a "retro gift", adding "the upbeat, brass-heavy track will certainly have you bouncing your bum". Music Times's Carolyn Menyes described the track as a notable departure from Trainor's previous work, writing "it differs greatly from the doo-wop influences of Trainor's first album and few hit singles" and compared it to the works of Christina Aguilera and Destiny's Child.
Sam Warner of Digital Spy liked the song's vibes, describing it as a "2000s banger", and "a typically Trainor-esque empowering anthem". He likened it to the early work of Britney Spears. Idolator's Carl Williott praised the song, he opined that Trainor was in "full flaunt mode", adding "she preens over a syncopated synth-horn-snare beat". Allison Weintraub of The Oklahoma Daily criticized the song, describing it as "terrible pseudo-rap", with "weirdly pluralized words".