Major General Andy Salmon will transfer authority for what will become Multi-National Division South to US Major General Michael Oates.
Most of Britain's 4,000 troops will leave by 31 May, the official end-of-combat date.
About 400 will stay after that, either in HQ roles or to train the Iraq Navy.
'Heads held high'
Major General Salmon says much has been achieved over the past six years.
He told the BBC: "We've helped deliver security, we've set the conditions for social and economic development and I think we can leave with our heads held high".
The US role in southern Iraq will be slightly different, focussing more on training the Iraqi police, and keeping open the supply route between the south and Baghdad.
The BBC's defence correspondent Caroline Wyatt says US soldiers are now a visible presence in Basra, with British troops handing over many of the buildings and duties at the camp as they pack up after six years in Iraq.
But Lt Colonel AJ Johnson, the American taking over the job of liaising with the Iraqi Army at Basra Operations Centre, says there will not be much difference in the US approach in Basra - which means ensuring the Iraqi Army and police remain the most visible presence on the streets here.
Lt Colonel Johnson told the BBC: "The bottom line, the aim of the transition itself is to make sure it's seamless and that there's generally no perception that the US army is here and they are going to do things different than the British did when they were here."
The Americans are also reducing their numbers, with two brigades due to leave the province of Al-Anbar, once the heartland of Al-Qaeda in Iraq.
However, the bulk of US troops are not due to leave until the end of 2011.
