Lamichhane became deputy prime minister for home affairs - heading the ministry that oversees identity cards - in a seven-party alliance that took power last month.
In its ruling on Friday, a five-member constitutional bench of the top court said the 48-year-old had contested November elections on an invalid citizenship certificate after abandoning his U.S. citizenship.
"He loses his ministerial position and there will be a by-election in his constituency," Lamichhane's lawyer Sunil Pokhrel told Reuters.
The departure of the minister was unlikely to affect the future of the ruling alliance, political analyst Krishna Khanal said.
Lamichhane, who hosted a popular television show before entering politics, could not be immediately reached for comment.
Pokhrel said Lamichhane will now seek to get a regular citizenship card and contest the by-election from the same constituency in south Nepal.
(Reporting by Gopal Sharma; Editing by Vin Shahrestani)
By Gopal Sharma
