Organisers of the week-long Welsh language festival in Abergavenny wanted to raise £300,000 but the final total will be significantly less.
Eisteddfod officials said the target was "ambitious".
The council said its contribution was an "investment well made".
In 2014, the local authority agreed to host the event and underwrite any shortfall as part of a £580,000 budget commitment.
Council leader Peter Fox told BBC Wales' Newyddion 9 programme: "We always realised there would be challenges in reaching the £300,000 target.
"This doesn't represent a great concern to us because this is a huge investment not only in celebrating Welsh language and culture but also Monmouthshire."
It is estimated as much as £6m will be spent in the local economy during Eisteddfod week.
Eisteddfod chief executive Elfed Roberts said: "We knew the target was fairly ambitious but the local working group has worked very hard.
"They have raised nearly £200,000 by organising numerous bilingual events.
"The council has supported us in many ways and sees the event as an investment in the county."
The Eisteddfod is returning to Abergavenny for the first time since 1913.
It will be held on the grounds of Castle Meadows between 30 July and 6 August.
Monmouthshire council will contribute £100,000 towards the cost of hosting the National Eisteddfod, after a fundraising campaign failed to reach its target.