The first generation Yasutsugu was born in Shimosaka village of Sakata district in Omi and called himself Shimosaka Ichizaemon. He moved to , served Yuki Hideyasu, and in his early years signed no Daijo Shimosaka (肥後大掾下坂). Between Keicho 10 and 11 (1605 to 1606) he was summoned to , forged before the two shoguns Tokugawa Ieyasu and Hidetada, and as a reward received the character from Ieyasu's name and the right to carve the Tokugawa hollyhock crest on his , taking the name Yasutsugu. The published sources repeat this biography nearly word for word, and state what the grant came to mean: the world calls him -Shimosaka (葵下坂) and Go- Yasutsugu (御紋康継) for the hollyhock cut above his signature. He served the shogunal house as its retained smith until his death in Genna 7 (1621).
No other of his day reads like his. The hollyhock is a granted crest carved above the signature, not part of the itself, and beside the signature he declared his material, the published record stating that he was then the only man boldly adding the words i nanban-tetsu (以南蛮鉄), made with nanban steel. Because he served and in alternate years, the sources state, he cut -ju (越前住) on home-province work and Oite Bushu (於武州江戸) on work, so the itself places and dates a blade. A third register is the Sunshu-uchi (駿州打), works forged at Sunpu while calling on the retired Ieyasu: only a few survive, the mostly long, their workmanship divided by the commentary into a refined and gentle type and a rougher, vigorously disordered one. One long Sunshu was among the Sunpu o-wakemono (駿府御分物), Ieyasu's keepsake division, and descended in the Kishu Tokugawa family. Practically everything he made is signed, seventy-two of the seventy-four designated works on record against a single unsigned blade, with Keicho dates and gold-inlaid cutting-test attestations on the finest.
His own manner begins in the steel. The forging is with standing out and a tendency for the grain to rise; forms thickly, fine enter, and the steel color carries a blackish tone, named outright by the published sources as the peculiarity of (越前がね). Over it the takes a shallow or as its base, with small running linked in series, entering frequently and mixed in. The is thick, with rougher standing in uneven patches; fine and run, vigorous and appear, at times to a -like effect, and the takes on the sinking tendency (沈みごころ) noted on blade after blade. The undulates shallowly, the point sharpening in what the texts call a -style manner, with and a long, strong return. The is the Keicho- build, wide with little taper, thick and shallow in , the point extended, the broad and . The commentary gathers all of this into a single phrase, the typical working range of the first generation Yasutsugu (初代康継の典型的な作域). His are deep, forceful and frequent, Fudo Myoo triads, , and among them, many read as Kinai-line carving.
The range divides by period and by purpose. Before the grant his work is rare: the published record states that signed no Daijo Shimosaka survive in no more than a handful, nearly all in the comparatively calm, well-ordered manner, gentler than the granted-crest prime; the texts set these early signatures beside those of the fellow smiths Sadakuni and Kanenori, so alike that attribution rests on the style. The opposite pole is the copy program. Because he re-tempered many of the Taiko's treasured blades burned in the fall of Osaka castle, he attempted reproductions of various celebrated , the sources stating that he was most skilled at the Sadamune copies. The , Ataki, Umetake and Shishi Sadamune copies survive, beside the Wakae Masamune, known otherwise only from Kotoku's drawings, the Ebina Kokaji of Munechika, whose re-tempered original is preserved in the Tokugawa Art Museum, the Namazuo Toshiro and Oyako Toshiro of Yoshimitsu, and a emulation of Hasebe. The 's judgment is explicit: in the and of his copies his own manner shows, and he does not idly imitate. The Namazuo Toshiro copy is faithful down to the placement of the , yet the blackish steel, linked and sinking betray the hand; where the model is lost, as with the Oyako Toshiro, the copy itself is held particularly precious. To the breadth belongs the konotegashiwa (児の手柏), the blade whose two faces are tempered in radically different manners, his example judged a model of the type.
are exceptional, for the record states that across the first and second generations production was extremely small; one of the few evokes old Norishige in its and and carries the only dragon-mounted Fudo carving in his work. His son, the second Yasutsugu, carried the manner so faithfully that the texts judge his work at times separable from the father's only by the form of the character tsugu, and a famous long taken for the third generation is now strongly argued to be the father's work under the second's proxy signature. After the second generation the family divided into the and Shimosaka houses, and the name prospered in both lines down to the end of the bakufu.
Fujishiro rates him Jo-jo . Seventy-four designated works stand on record: two Important Cultural Properties, thirteen Bijutsuhin, twelve and forty-seven , so that fifty-nine blades occupy the and tiers. Thirty blades carry recorded provenance, and the names trace his patronage. Honda Hida no Kami Narishige, the castellan and his great patron, left possession inscriptions on a number of his best pieces; the Tokuju Shishi Sadamune copy keeps its original Honda-crested . The with the go Furaijin (風雷神), carried by Matsudaira Tadamasa at the Summer Siege of Osaka, was long transmitted in the Matsudaira house; other blades passed through the Owari Tokugawa Reimeikai Foundation, Mitsui Takakimi and Enomoto Takeaki. He has no blade in the National Treasure class and his Important Cultural Properties are few, so what a collector meets is the signed and body of and , which comes to the market from time to time; the named- copies, the Sunshu-uchi and the pieces bearing the Honda possession inscriptions surface only rarely, each an event for the field when it does.