Clashes were also taking place in nearby Sadad, a mostly-Christian town.
The latest development comes amid air campaigns in Syria by Russia and a US-led coalition.
IS has been expanding from its mainly northern and eastern strongholds towards Homs in central Syria in recent months. The group overran the town of Tadmur - home to the ancient ruins of Palmyra - in May, and later took al-Qaryatain town in August.
The latest offensive on Maheen and Sadad brings IS to within 20km (13 miles) of the main road that links the Syrian capital Damascus to Homs and other cities further north.
Christian population
The Observatory said at least 50 government soldiers were killed or wounded in the fighting. The attack on Maheen began late on Saturday with twin suicide car bombs, a favoured tactic for IS militants launching an assault.
By Sunday morning the Observatory reported that the whole town was reported to be in IS hands. An IS statement also said the group had taken the town.
Maheen is home to a large military complex and arms depot.
Meanwhile, clashes between government troops and IS are said to be continuing on the outskirts of Sadad. The town is home to Syria`s Assyrian Christian minority, where the ancient language of Aramaic is still spoken.
It comes amid continued Russian air strikes in support of Syrian President Bashar al-Assad, which Russian officials say are targeting IS and other "terrorist groups".
However, activists on the ground say the strikes have been hitting moderate rebels and civilians in western areas, where IS have little or no presence.
They said more than 60 people were killed by Syrian army raids and Russian strikes in the northern province of Aleppo on Saturday.
Human shields
On Friday, more than 70 people were reported killed and hundreds more wounded in an air strike and shelling on a market in the rebel-held Damascus suburb of Douma.
In an attempt to ward off the attacks, rebel groups in Douma are reportedly using captured soldiers and other people associated with the government as human shields.
Video has emerged of men and women in large metal cages on the back of lorries being driven slowly through the streets. One of the captives says he is an army officer, and he appeals for an end to bombing in the area.
The US-led coalition, which is also hitting IS targets in Syria, said on Sunday it had conducted nine air strikes across the country, including in Mar`a and al-Hawl, in the north.
This week the White House announced that fewer than 50 US special forces troops would be sent to Syria to assist anti-government rebels in fighting IS.
Syria-UN meeting
Separately on Sunday, Syrian Foreign Minister Walid Muallem met UN peace envoy Staffan de Mistura in Damascus to discuss ongoing international talks on the Syria conflict.
Mr Muallem said "important points" were made during talks in Vienna on Friday between Syria`s key allies (Iran and Russia) and opponents (the US, Saudi Arabia, Turkey), aimed at pursuing a new peace effort involving the Syrian government and opposition groups.
The four-year-old war in Syria, which began with an uprising against President Assad, has left 250,000 people dead and forced half the country`s population - or 11 million people - from their homes.
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