Miss Mills and Sir Paul join a growing list of celebrities who have given their support to the campaign, among them Diana, Princess of Wales, who visited Angola.
But former President Clinton decided that land mines were a necessary deterrent that protected South Korea from the North and declined to sign up to the treaty.
Miss Mills, who lost her left leg below the knee after a road accident, heads a charity which provides limbs for victims of wars around the world.
Scattered mines
About 100 million mines are thought to be scattered in minefields across the world and 26,000 people are maimed every year.
Miss Mills and Sir Paul join a growing list of celebrities who have given their support to the campaign, among them Diana, Princess of Wales, who visited Angola.
After the meeting at the State Department in Washington, Sir Paul said: "I expressed the point of view that even brave soldiers who defend the country... dislike the idea of leaving the war behind them and causing grief for civilians. The secretary [of state] was very understanding about that."
Heather Mills said: "I have worked with the people. I've lived in the former Yugoslavia and the reality of a full ban which we all want is many, many years off so I am looking for an interim solution.
Discarded landmines
"I want to be able to go back and tell people if another war starts up like it did in the former Yugoslavia, that it will be safer to go out when it is over."
Mr Powell declined to back the treaty but did say there were "many areas in which we can co-operate".
The secretary of state expressed his support for the Adopt-A-Minefield scheme, which is working to clear discarded landmines around the world.
The organisation buys minefields for $25,000 (£17,000) a plot and clears them.
Smart landmines
Miss Mills said she had pressed the US government to use so-called smart landmines which deactivate themselves and do not need to be cleared and destroyed.
The US currently has a stockpile of about 11 million landmines.
Mr Powell said the US was already working to help clear landmines around the world.
"We have contributed something like £350m (£242m) over the last seven years," he said.