The formation of the cabinet brings to an end almost three weeks of political upheaval by politicians over key posts and what shape the cabinet should take.
Siniora's government, which was approved by President Emile Lahoud after three draft lineups had been rejected, is the first since May-June elections gave opposition groups a majority in parliament for the first time since the civil war.
"We have reached agreement on a 24-member government," Siniora said after mandatory consultations on the cabinet with head of state Lahoud.
The new cabinet did not include associates of Christian leader and former army General Michel Aoun, who returned to Lebanon after 14 years of exile in France.
He had insisted on the justice portfolio but was refused and so declined to join the government.
Even though Hizb Allah patrols the tense Lebanese border with Israel in the formerly Israeli-occupied south, and is regarded in Lebanon as a legitimate resistance to Israel, Washington and Paris have insisted on Hizb Allah's disarmament and continue to classify it as a "terrorist group".
The Shia Muslim Hizb Allah group is represented in the new government by Mohammed Fneish, who becomes energy minister, while Hizb Allah ally Tarrad Hamadeh retained the post of labour minister.
A Hizb Allah representative was
appointed minister of energy
The US, meanwhile, warned on Tuesday that it would reject any contacts with Fneish.
"To the extent that there were members of Hizb Allah, active members of a foreign terrorist organisation that are present in the government, our ability to deal with those individuals is circumscribed by law," said Adam Ereli, US State Department spokesman.
"We have a policy toward Hizb Allah. It's clear. It hasn't changed," he added.
Other cabinet appointees include Shia independent Fawzi Salloukh, who becomes foreign minister.
The former veteran diplomat who served for more than three decades with the foreign corps does not belong to either group but is considered an acceptable choice by both.
Under Lebanon's political system, the country's prime minister, who is always drawn from the Sunni community, must gain the approval for his government from the president, a post reserved for Christians.
The government will incorporate members of a 72-strong coalition, including supporters of the Future Movement of slain former prime minister Rafiq al-Hariri's son Saad al-Hariri, Druze leader Walid Jumblatt and some Christian MPs.
The government will be led by members of Siniora's main opposition bloc - led by Saad al-Hariri - which has an eight-seat majority in the 128-member parliament.
The Hizb Allah-Amal alliance has 30 MPs.
The cabinet includes 12 Christians and 12 Muslims, according to Lebanon's sectarian political system.
"I have great hope that this cabinet will function as a united working team ... to confront the great challenges facing us," Siniora said following the announcement.
He said he was proud of Fneish's participation in the ministry and promised that the new cabinet would work on improving relations with Syria, which have suffered since Syrian troops withdrew in April under US and UN pressure as well as massive street protests in Beirut.
Elias al-Murr (L) survived an
assassination attempt on 12 July
The new administration includes ministers close to Lahoud, including his son-in-law, former defence minister Elias al-Murr, who survived an assassination attempt last week.
He retains the defence portfolio.
Charles Rizk, a former information minister, becomes minister of justice, a sensitive position as the new government considers the recommendations of the UN investigation into al-Hariri senior's murder.
Siniora, a Sunni Muslim, was the faithful right-hand man in both business and politics of Rafiq al-Hariri.
He oversaw al-Hariri's huge Arab banking interests and ran the nation's finances in all five of the governments the late tycoon headed between 1992 and 2004.
Siniora has pledged to rebuild national unity and embark on sweeping reforms to revive the debt-laden economy.