She criticised the Welsh Government's response to the referendum result, suggesting there was no sense of urgency.
Ms Wood said the option of Welsh independence should be considered if Scotland leaves the UK.
The Welsh Government said it was doing everything it could to strengthen the economy after the EU poll.
Wales voted 52.5% to 47.5% to Leave the European Union at the June poll.
"The Welsh Government should develop and prepare an official Welsh negotiating position for EU withdrawal," Ms Wood said.
"That position should be scrutinised and agreed by the National Assembly, so that it is in place in time for an incoming UK prime minister."
"As soon as that prime minister sits down at the desk in 10 Downing Street, the Welsh position should be in her or his in-tray."
In a speech in Cardiff, Ms Wood offered to work with the Labour-led Welsh Government, saying she was "more than willing to pool Plaid Cymru's talent, ambition and energy".
She called for a national mitigation plan, and said a Welsh negotiating position should be in place by the time a successor to David Cameron arrived at 10 Downing Street.
The Welsh Government had failed to come up with a contingency plan for a Leave vote, she said.
"Where is the sense of urgency? Where is the sense of emergency? Where are the delegations to Brussels and other European capitals?" she said.
Ms Wood added: "The new situation would necessitate people here deciding whether we wanted to be part of an England and Wales entity, or whether we want to do something different."
A Welsh Government spokesman said: "Following the result of the referendum, we are doing everything within our power to protect Welsh interests, strengthen the economy, and unify the nation.
"The first minister has made very clear to the prime minister that Wales should be fully involved in negotiating the UK's exit from the EU, and will ensure the interests of Wales and its people are taken fully into account so that we achieve the best possible deal."
The assembly should approve a Brexit plan for Wales, Plaid Cymru leader Leanne Wood has said.