Abstract Recent discoveries of double neutron star (DNS) mergers and ultra-stripped supernovae (SNe) raise the questions of their origin and connection. We present the first 1D model of a DNS progenitor system that is calculated self-consistently until an ultra-stripped iron core collapse. We apply the MESA code starting from a post-common-envelope binary consisting of a 1.35 M ⊙ NS and a 3.20 M ⊙ zero-age main-sequence helium star and continue the modeling via Case BB Roche-lobe overflow until the infall velocity of the collapsing iron core exceeds 1000 km s −1 . The exploding star has a total mass of ∼1.90 M ⊙ , consisting of a ∼0.29 M ⊙ He-rich envelope embedding a CO core of ∼1.61 M ⊙ and an iron-rich core of ∼1.50 M ⊙ . The resulting second-born NS has an estimated mass of ∼1.44 M ⊙ , and we discuss the fate of the post-SN system, as well as the mild recycling of the first-born NS. Depending on the initial conditions, this family of systems is anticipated to reproduce the DNS mergers detected by the LIGO network.