The sale includes paintings, a historic library, antique jewellery, and a vintage motorcycle.
The money will be used to restore the Glyn Cywarch estate in Talsarnau.
Lord Harlech inherited the estate when his father, former Conservative peer the 6th Baron Harlech, died in February.
Lord Harlech said of the restoration: "It's a big undertaking, but we'll get it done."
He added: "Just as my family collected fine art and antiques over the generations, it now feels the right time to find a new generation of collectors to enjoy them."
More than 400 items will be listed at the auction which will take place in London on 29 March, 2017.
Leading items include a 1936 Lagonda Rapier sports car which is valued between Â£20,000-Â­25,000, and personally inscribed manuscripts by Jackie Kennedy, wife of the former US President John F Kennedy.
The contents of a Grade II-listed estate in Gwynedd which belong to the Harlech family will be sold at auction.