Girija Prasad Koirala, Nepal's prime minister, who administered the oath of office on Monday, said: "Now we will all concentrate on the election."
In a press conference following the swearing in ceremony, Mahara said: "We have joined the cabinet mainly to conduct the election on time.
"Our party is ready to fight the hurdles that will emerge in the days ahead in successfully conducting the poll.
"Our way of working will be different now on," Mahara said.
The Maoists, who fought a 10-year insurgency against the government, in which more than 13,000 people died, agreed to rejoin the interim coalition government last week.
The reconciliation came only after the seven main political parties agreed to abolish the country's 239-year-old monarchy immediately after elections now scheduled for April.
The Maoists had withdrawn their political involvement in September demanding electoral reform and the removal of the king.Four of the five newly appointed cabinet ministers had served in the government between April and September.
The government then postponed elections which had been planned for November.
Pampha Bhusal is the only one new to the cabinet, and will be the minister for women and social welfare.
Dev Gurung is to be the minister for local development, controlling city and village councils.
Matrika Yadav will be the minister for forest and soil conservation.
Hisila Yami, who becomes the minister for physical planning, said: "We fought for a constituent assembly for 10 years. It will be our priority to make the election to the assembly a success.