No redundancies will be made, with all of Lexis' 30 staff moving over - with the exception of Dominic Shales, who joined Lexis when it acquired his agency Paratus in 2012 and will leave next month.
The Lexis brand will disappear in September, but the merger is effective immediately. Text100's UK MD Tara O'Donnell remains at the helm of the Next Fifteen-owned agency, which now numbers 100 staff.
"The time was right for me personally to move on," Shales told PRWeek, adding: "It was all amicable and I'm completely behind the strategy - I think it is the right thing to do."
Lexis deputy MD Ruth Kieran takes the same role in the combined agency, while Lexis' Toby Conlon, Bianca Lee-Chang and Virgine Luce become the respective heads of corporate reputation, consumer and digital marketing.
Aedhmar Hynes, Text100 Global CEO, said: "Adding Lexis’ stellar consumer brand experience further strengthens Text100 as a global marketing communications agency, building on the previous acquisitions of Incredibull [in 2015] and Republic Publishing [in 2014], which were successfully merged into Text100 last year."
Text100 moved into its office in the central London district of Holborn - having previously been in Hammersmith - at the end of 2015. Lexis moved into the same offices 12 months later.
Hynes told PRWeek that the plan was for the agencies to work together more closely, and that it then became clear that a merger would make sense. "Both agencies were moving on that same path - to become more integrated marketing agencies," she said.
She also said that Tim Dyson, the CEO of parent company Next Fifteen, did not want the move to feel forced. "In Tim's mind, it had to be something that both agencies wanted to do, rather than something that was done to them," said Hynes.
She said the merger was "more about expanding the remit of current clients than acquiring new clients - we’re getting more and more opportunities with marketing departments rather than just comms departments", and said that she understood this was a strategy common to other global agencies.
Hynes also said it "would have been great" to keep Shales, adding: "We wish him all the best."
Major clients of what was Lexis include DHL, Pernod Ricard and Harley Davidson - and previously Avios. Text100 has briefs with Avios and DHL, as well as others including Vodafone and Sophos.