The attack occurred on Thursday during morning prayers Malele area, about 25 kilometers from the Dadaab refugee camp.
Witnesses told police eight attackers were armed with AK47 rifles.
Police confirmed the dead included five men and two women. The men were killed inside the mosque while the women were shot outside.
Police say they are yet to establish the motive of the attack and if it is linked to terrorism or local inter-clan feuds.
Locals said scores of people are missing after the attack.
After the shooting, the attackers burnt houses near the mosque before escaping.
Garissa County Commissioner Maalim Mohamed confirmed the attack and said his officers had been dispatched there to pursue the case.
North Eastern has suffered the majority of terror related attacks that have occurred in the country in the past year.
The most recent incident was last Saturday night where a terrorist was killed as he tried to set up a bomb at a stadium that was to be used by Narc Kenya presidential candidate Martha Karua the following day in Garissa.
In the past year, there have been over 40 attacks involving grenades or explosive devices in Kenya, leaving at least 100 people dead and around 220 people injured.
At least 15 of these attacks occurred in North Eastern Province, mainly in Dadaab, Wajir, and Garissa and four in Mombasa.
Six grenade and improvised explosive device (IED) attacks have occurred in Nairobi, illustrating an increase in the number of attacks and an advance in the sophistication of attacks.
The attacks came after the Kenyan troops moved into Somalia to hunt down Al Shabaab militants who are blamed for several attacks in the country.