The Fine Gael leader will step down as Taoiseach and party leader before the next Irish General Election.
Mrs Long said Mr Varadkar had a major role to play in positive north-south relations.
“Northern Ireland and the south have a good relationship, more so than at any time in recent history, which is due in no small part to the work of Leo Varadkar and other members of the Irish Government,” she said.
“In terms of local politics, he has played his part in helping restore the Executive here twice and also contributed greatly to many challenges we have faced such as the fall-out of Brexit, the issue of legacy and has also added his voice to the chorus of those recognising the need for reform of the Stormont institutions.
“On a personal level, I always enjoyed meeting and co-operating with Leo on a wide range of topics, and he always showed a genuine interest in local issues. I wish him the best in whatever he decides to do now.”