Despite Ren's high status, little is firmly established about his background or career except for the time that he served as chancellor—as, unusual for a chancellor, he did not have a biography in either the Old Book of Tang or the New Book of Tang. He was not even listed as a chancellor in the table of the chancellors' family tree, among the Rens.

As of 691, Ren was serving as Fengge Shilang (鳳閣侍郎), the deputy head of the legislative bureau of government (鳳閣, Fengge), when Wu Zetian gave him the designation Tong Fengge Luantai Pingzhangshi (同鳳閣鸞臺平章事), making him a chancellor de facto. In 692, he, along with fellow chancellors Di Renjie and Pei Xingben and other officials Cui Xuanli (崔宣禮), Lu Xian (盧獻), Wei Yuanzhong, and Li Sizhen (李嗣真), were accused of treason by Wu Zetian's secret police official Lai Junchen. Eventually, they escaped death when Di was able to hide a secret petition inside clothes that his family members took back home and was able to persuade Wu Zetian that they were not plotting treason. However, they were still exiled—in Ren's case, he was demoted to be the magistrate of Jiangxia County (江夏, in modern Wuhan, Hubei). There was no further reference in history about him, and it is not known when he died.

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儘管任知古身居高位，但他的家世和拜相前的仕途幾無記載。作為宰相，《舊唐書》和《新唐書》反常地都沒有他的傳，他甚至沒有被列入《新唐書·宰相世系表》中的任氏表。僅知垂拱三年（687年）任朝議郎、行麟台郎，並作《寧義寺經藏碑》。

天授二年六月庚戌（691年），任知古在鳳閣侍郎任上被武則天授同鳳閣鸞臺平章事。

天授三年（692年）一月（一作二月、十月），他和宰相地官侍郎狄仁傑、冬官侍郎裴行本、司禮卿崔宣禮、前文昌左丞盧獻可、御史中丞魏元忠、潞州刺史李嗣真被武則天手下酷吏御史中丞來俊臣彈劾謀反。來俊臣奏請降敕，一問即承認謀反者可以減免死罪，並以此誘惑任知古等承認謀反。由于狄仁傑將一份表章藏在衣物中讓家人帶回，得以向武則天澄清眾人並未謀反，武則天說：「古人以殺止殺，我今以恩止殺。就聽群公為知古等所請，賜他們再生，各授以官職，以觀後效。」來俊臣與文昌左相武承嗣、侍御史霍獻可、鳳閣舍人向光道、侍御史張知默都請求誅殺任知古、狄仁傑等，武則天不許。于是眾人免于一死，但仍被外貶。任知古被貶為江夏縣令。

唐玄宗先天二年（713年），唐玄宗發動先天政變，誅殺政敵皇姑太平公主一黨。時太上皇唐睿宗聽聞鼓譟聲，召宰相郭元振升承天樓，宣詔閉關自守，時任侍御史的任知古在朝堂招募數百人于朝堂，被其攔住不能入內。

開元初年，他仗著官威在朝上詬罵官員，右拾遺嚴挺之很責怪他不敬，反遭御史台彈劾被貶為萬州員外參軍。

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