The 22-year-old matched her fellow British number one Andy Murray as she brushed aside Croatian Tomljanovic 6-3 6-2 on Court Three, joining Naomi Broady in round two of the women's singles.
Watson arrived at Wimbledon fresh from a run to the semi-finals at Eastbourne, which lifted her ranking up to 60th.
In February she was ranked 100 places lower, a legacy of the glandular fever she struggled to overcome all of last season.
This year has been a completely different story, with the British number one reaping the benefits of good health and a more aggressive approach under coach Diego Veronelli.
Tomljanovic, 21, had never won a match at Wimbledon but is one of the rising talents of the game and defeated Agnieszka Radwanska on her way to the fourth round of the French Open.
The first set was a story of chances taken and chances missed, with Watson converting her only break point in the second game and saving all four break points she faced.
Tomljanovic certainly possesses the power to be a danger on grass but Watson refused to be pushed back and the Croatian began to go for too much.
Watson was not lacking in the power stakes either, hitting 17 winners to her opponent's 16, with one forehand down the line in the fifth game of the second set in particular an indication of the strides she has made.
By that stage she was already a break up and she broke again before winning a fifth straight game to move to the brink of victory.
Tomljanovic, ranked at number 53, forced Watson to serve it out, which she did with aplomb to set up a second-round meeting with either ninth seed Angelique Kerber or Urszula Radwanska.