Mutants of lamin A cause diseases including the Hutchinson-Gilford progeria syndrome (HGPS) characterized by premature aging. Lamin A undergoes a series of processing reactions, including farnesylation and proteolytic cleavage of the farnesylated C-terminal domain. The role of cleavage is unknown but mutations that affect this reaction lead to progeria. Here we show that interphase serine 22 phosphorylation of endogenous mutant lamin A (progerin) is defective in cells from HGPS patients. This defect can be mimicked by expressing progerin in human cells and prevented by inhibition of farnesylation. Furthermore, serine 22 phosphorylation of non-farnesylated progerin was enhanced by a mutation that disrupts lamin A head to tail interactions. The phosphorylation of lamin A or non-farnesylated progerin was associated to the formation of spherical intranuclear lamin A droplets that accumulate protein kinases of the CDK family capable of phosphorylating lamin A at serine 22. CDK inhibitors compromised the turnover of progerin, accelerated senescence of HGPS cells and reversed the effects of FTI on progerin levels. We discuss a model of progeria where faulty serine 22 phosphorylation compromises phase separation of lamin A polymers, leading to accumulation of functionally impaired lamin A structures.