Jo Swinson has vowed she would not be “giving up” following talks with a fringe meeting at the party’s Bournemouth conference.
She said temporary post-exit trade arrangements would help keep Britain linked to EU membership.
Ms Swinson said a transition deal after Brexit would give the country breathing space to think twice after the UK’s planned EU withdrawal in March 2019.
“What transition periods also do is they give time,” she said.
“They mean that it is not this mad rush and there is the opportunity there for the country to take stock and to think.
“The closer we are to our EU neighbours, the easier it is, at some future point, to reverse the decision.
“Because, if you haven’t then hugely diverged from everything, if you have still got the same rules in place, if you have still got the same trading arrangements, then actually the coming back becomes much easier.
“So, I think, strategically, that is important for us to try to achieve working with others.”
Chancellor Philip Hammond said the Government is looking for a transition arrangement for a set amount of time that would “look a lot like the status quo”.
Lib Dem leader Sir Vince Cable said: “We are not in the business of trying to help the Government find an easy soft Brexit because it’s not real.
“We are for Remain. We believe Britain should stay in the European Union. Period.”
Sir Vince said he believed there needed to be another Brexit vote, which he said would be “the first referendum on the facts.”
Brexit is expected to be top of the agenda during the four-day conference and will look closely over fears over its impact, public services and NHS staffing levels.
Senior Lib Dem MP Norman Lamb has warned the Lib Dems are in danger of becoming a single issue party by they are focusing so much on Brexit.