It has been reported that at least 350 jobs would be lost, but Carillion said that redundancy figures were as yet unknown.
"We announced the closure on Monday and started consultations on Friday so it's too early to say," said a Carillion spokesman.
"We are holding individual consultations as to whether people want to move to a role elsewhere or whether they want to stay in Lanarkshire."
Carillion completed the £561M takeover of Alfred McAlpine in February this year.
The office was highlighted as surplus to requirements during a review into Carillion office space following the contractors' merger. "There is some duplication of head-office functions such as finance, IT and HR," said the spokesman.
"We are part way through a review looking at Carillion office space in general and can't say for definite that there won't be more closures."
Having assessed the viability of Alfred McAlpine's head office, Carillion's post-takeover review will now focus on the purchased contractor's remaining 33 offices in the UK.
They overlap with Carillion offices in many regions of the country, including Manchester, Bristol and the Midlands.
When Carillion bought Mowlem for £313M in 2006, it closed the contractor's head office in west London, but kept the firm's Leeds office, home to Mowlem's Facilities Management business.