The British pair scored 308.16 to beat Russia's Yulia Timoshinina and Victor Minibaev (300.30) and Italy's Noemi Batki and Maicol Verzotta (299.58).
Toulson, 17, won individual 10m gold on Wednesday.
Britain have won three golds after Ruby Bower and Phoebe Banks won the women's 10m synchro on Thursday.
Noah Williams and Matthew Dixon also took bronze in the men's 10m synchro on Thursday.
Lois Toulson won her second European Championship gold with victory alongside Matty Lee in the mixed 10m synchronised event in Kiev.