Despite his ideology, Guatemala's new leader said he doesn't want to be identified with other leftist governments in Latin America, including that of Hugo Chavez, the Venezuelan president.
Colom has said each country must "find its own path", and that he won't accept Venezuela's offer of oil for preferential terms until he has consulted with his country's business elite.
The ceremony was attended by Mayan leaders, some wearing colourful embroidered blouses and skirts. Also taking office on Monday was Rafael Espada, the vice-president.
"We are going to fight for the unity of the country, for the harmony with our indigenous people," Colom said.
Colom, an industrial engineer, has promised a broad social agenda that includes building schools and medical centres, creating jobs, and bringing security to a country where gangs behead victims and drug traffickers control much of the police forces.
But even Colom has recognised his job won't be easy. Half of Guatemala's 13 million people live on less than $1 a day, and discrimination against the ethnic Mayan majority is rampant.