He successively served three military governors (Jiedushi) stationed at You Prefecture — Xue Chuyu (薛楚玉), Zhang Shougui (張守珪), and An Lushan, and because of his accomplishments received repeated promotions, eventually receiving a title as general and forward commander at one of the circuits that An governed, Pinglu Circuit (平盧, headquartered in modern Chaoyang, Liaoning).
The military governor of Pinglu, Lü Zhihui (呂知誨), was a follower of An's, and he trapped and killed the deputy Protectorate General to Pacify the East, Fumeng Lincha (夫蒙靈詧).
In response, Dong Qin and his colleagues Liu Kenu (劉客奴) and Wang Xuanzhi (王玄志) killed Lü and took over the circuit, remaining in remote contact with Yan Zhenqing, one of the few Tang generals resisting Yan forces north of the Yellow River.
The Pinglu army subsequently made a number of harassing attacks against An's original base at Fanyang (范陽, i.e., modern Beijing), and Dong distinguished himself in these battles, including a victory over the Xi general Abuli (阿布離) when the Xi allied with Yan.
In spring 757, Wang sent Dong with an army over the Bohai Sea, on simple rafts, to join the general Tian Shen'gong (田神功) to attack Pingyuan (平原, roughly modern Dezhou, Shandong) and Le'an (樂安, roughly modern Binzhou, Shandong) Commanderies, recapturing them from Yan forces.
The Tang general in overall command in the area, Li Xian (李銑), exercising imperial authority, made Dong the governor of Pingyuan Commandery.
A number of Tang generals, including, in addition to Dong Qin, Guo Ziyi, Lu Jiong (魯炅), Li Huan (李奐), Xu Shuji (許叔冀), Li Siye, Ji Guangchen (季廣琛), Cui Guangyuan (崔光遠), Li Guangbi, and Wang Sili (王思禮), converged on Yecheng and put it under siege, although Tang forces were unable to capture Yecheng quickly.
Dong, along with Tian, Liang Pu (梁浦) and Liu Congjian (劉從諫), served under Xu.
Emperor Suzong made Dong the military commander of the Shanxi (陝西) and Shence (神策) Armies, and bestowed on him not only the imperial surname of Li, but also a new personal name, Zhongchen ("faithful subject").
Soon thereafter, Li Zhongchen was made the military governor of Huaixi Circuit (淮西, headquartered in modern Zhumadian, Henan), governing over 12 prefectures.
In 767, as the military governor of Tonghua Circuit (同華, in modern Weinan, Shaanxi), Zhou Zhiguang (周智光), became increasingly violent (including, among other things, slaughtering the entire household of fellow military governor Du Mian (杜冕)) and threatening to the emperor, Emperor Daizong sent Guo to attack Zhou, whose subordinates subsequently killed him and surrendered.
In 771, Li Zhongchen personally participated in the yearly defense rotation on the western border against possible Tufan attacks.
In 775, there was a disturbance at Shan Prefecture in which the soldiers mutinied and forcibly expelled their military commander, Zhao Lingzhen (趙令珍).
The circuit governor (觀察使, Guanchashi) Li Guoqing (李國清) could not suppress them and only was spared by humiliating himself before them.
Emperor Daizong ordered Li Zhongchen, then on another trip to Chang'an to pay homage to him, to suppress the mutiny.
When Li Zhongchen arrived at Shan Prefecture, the soldiers knew his reputation and submitted to him, not daring to do anything else.
It was said that Ma knew that Li Zhongchen would credit only himself with the victory, and so did not enter Bian Prefecture in order to avoid a conflict.
Emperor Daizong subsequently gave Li Zhongchen the honorary chancellor designation of Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事) and issued orders adding Bian Prefecture to his circuit and moving Huaixi's headquarters to Bian Prefecture.
Li Zhongchen interceded and stated, "Your Imperial Majesty is the Son of Heaven, but you allowed your teacher to commit crimes out of poverty.
Emperor Dezong was about to approve the execution, when Li Zhongchen stated, "Xin Jinggao should have died long ago."
In fall 783, soldiers from Jingyuan Circuit (涇原, headquartered in modern Pingliang, Gansu), were at Chang'an, ready to be deployed to the east to battle four rebellious military governors — Zhu Tao, Wang Wujun, Tian Yue (who had inherited his post from Tian Chengsi), and Li Na.
Angry that they were not given rewards while at Chang'an, they mutinied, forcing Emperor Dezong to flee to Fengtian (奉天, in modern Xianyang, Shaanxi).