Mondrian died on February 1st 1944 and so the 70th anniversary of his death was in 2014. Copyright to Mondrian's works is/was held by the Mondrian / Holtzman Trust. The linked page states
Reproduction rights and Mondrian
Seventy years after an artist’s death their images enter the public domain and no longer require copyright permission. Piet Mondrian died in February 1944 and after January 1, 2015 his images are in the public domain. In Spain the duration of the copyright is longest, being valid the term of 80 years for authors deceased prior to December 7 1987.
In the US copyright protection is extended for images by foreign artists created and first reproduced and published in another country between 1923 and 1978. The extension is for 95 years from that first publication date based on the 1996 Uruguay Round Agreements Act. 35% of Piet Mondrian’s images created between 1911 and 1944 now have copyright expiration dates between 2019 and 2061 depending on the date of first publication. *
We recommend that you contact us about images you wish to reproduce which may be available in Spain in the US or on the internet so that we can check on the copyright status and clear rights if needed.
US copyright and Mondrian
The Trust, which holds the US copyrights in the images of Piet Mondrian, authorizes reproductions of images by Mondrian in publications in English which may be available in the US, and on the Internet.
Licenses are granted for reproductions of Mondrian images incorporated in commercial products such as apparel, home textiles, rugs, furniture and giftware to be distributed and sold in the US. Royalties and advances are paid to the Trust based on a licensing agreement.
Copyright permission is given for reproductions of one or more Mondrian images in books, auctions catalogues, museum publications, websites, and other editorial uses. Copyright fees are due when permission is granted.
To discuss a product license or copyright permission, click on the contact us button or send an Email to email@example.com
* I copied that paragraph from the Trust's page on 20th March 2015. Looking at it again on 27th May, it now states "45% of Piet Mondrian’s images created between 1911 and 1944 now have copyright expiration dates between 2019 and 2061 depending on the date of first publication".
The Trust recently circulated Mondrian scholars with a list of the works affected.
My attention has also been drawn to some interesting material on the College Art Association website, including this document, Copyright, Permissions, and Fair Use among Visual Artists and the Academic and Museum Visual Arts Communities.
[9th May] There's a New York Times article on the subject here that justifies my decision not to publish.